Woof woof
 

"TonyL"
Phil Allison

There * IS * a benefit to be had using two, low cost woofers by
mounting them face to face and connecting the terminals out of
phase. The combined unit has the same resonant frequency but twice
the cone mass with the same cone area, twice the power handling and
usually much better linearity that one woofer does. The box can be
half the volume too.

This idea is known as " push pull" or "Isobaric".


Thanks Phil, just looked it up. Seems like a good way to 'use up' my two
drivers.


** You should download WinISD and try it - it is just sooooooo simple to
use.

http://www.linearteam.dk/default.aspx?pageid=winisd

There is a large library of various brand woofers included so you can
immediately use the supplied TS data.

Also, and most importantly, you can vary box volumes and tuning points at
whim and instantly see what the effect is.

In most cases, major reductions in box volume are possible with only small
penalties in flatness and LF cut off points.



...... Phil

Woof woof
 

In message , TonyL
writes
Don Pearce wrote:
On Sun, 08 Apr 2012 16:46:17 +0100, Jim Lesurf
wrote:

In article , TonyL
wrote:
Arny Krueger wrote:

At some point, measuring T/S parameters starts looking good.


Better than building 3 boxes, certainly.

I don't know about the UK, but in the states you can get a USB
gizmo with software that does full relevant set of driver tests
using your PC for under $100.


OK, but that would be somewhat more than I paid for the drivers. I
do have an amp module that goes down to DC as well as other bits of
audio gear so I might have a go at T/S parameters to give me a
ballpark figure for the box volume.

Someone else will know the details rather better than myself. But I
think you can estimate some of the basic parameters for a large bass
speaker from some quite simple measurements.

e.g. By using a ruler to measure the displacement of the speaker
cone when you apply a fixed dc voltage. And - with the speaker
pointing upwards - how much displacement a small weight produces.
(Rotating from vertical to horizontal and measuring the displacement
will also let you estimate the mass of the cone, etc.) None of that
may be very accurate, but may still be close enough.

IIRC The "Loudspeaker Design Cookbook" by Vance Dickason explains
this in an appendix. But I may have actually read the details
somewhere else. Not really my topic.

Slainte,

Jim

I've found a very good explanatory web page which describes in fair
detail how to measure the T/S parameters. There is a downloadable
spread sheet to put the measurements in and produce a full T/S
parameter set.

http://sound.westhost.com/tsp.htm

I've fed some dummy data in, and the answers are very believable.


Thanks all. I'll drag one of the units from the loft and try the T/S 'added
mass' measurement method, not having a ready made box yet.

Great site, BTW.

Anybody like to comment on how T/S params translate to box volume if I tried
an isobaric design as mentioned by Phil? Just divide enclosure volume by 2?





If you are using the 'added mass' method to measure the TS parameters,
make sure that the mass itself can't move about on the speaker cone. If
this happens you can get a very noisy measurement and poor accuracy. I
have one of those small digital scales which I use to weigh out a
quantity of Blu-Tak (or whatever the equivalent is where you are). I
then form this into a ring which I (carefully!) press into place around
the boundary of the dust cap and the cone. This works well and allow
the measurement to be performed with the L/S vertically aligned,
although I think that the alternative method of measuring in free air
and then in a box of known volume is more accurate. The added mass
method should work well enough for your needs though. Perform the
measurements a few times to act as a 'sanity check'! (I use the 'LIMP'
program from the 'ARTA' suite to measure L/S parameters and get adequate
results).
--
Chris Morriss

Woof woof
 

"TonyL" wrote in message
...
Arny Krueger wrote:

Given the unknown nature of these drivers, there is very little that
you can reasonably assume if your goal is *not* a boom box.


Fair enough...which is why this project is not so serious and might not
even happen. I am just looking for opinions at this stage.

Are there any clues from a physical inspection that might indicate what I
can expect?

Well, you could build 3 boxes with a 3:1 range of sizes and see
which one sounds best.

At some point, measuring T/S parameters starts looking good.


Better than building 3 boxes, certainly.

I don't know about the UK, but in the states you can get a USB gizmo
with software that does full relevant set of driver tests using your
PC for under $100.

OK, but that would be somewhat more than I paid for the drivers. I do have
an amp module that goes down to DC as well as other bits of audio gear so
I might have a go at T/S parameters to give me a ballpark figure for the
box volume.

I think your economic analysis could be improved upon. ;-)

The whole purpose of this project appears to be to create something with
significant value.

The economic justification for project expenses should be based on the value
of the expected result, not the materials cost.

Also, economics probably does not actually have a lot to do with this
project. The greatest value to you would be pride from creating of something
with significant value.

Same argument, two different approaches that probably lead to the same
actions... ;-)

Woof woof
 

Phil Allison wrote:
"TonyL"
Phil Allison

There * IS * a benefit to be had using two, low cost woofers by
mounting them face to face and connecting the terminals out of
phase. The combined unit has the same resonant frequency but twice
the cone mass with the same cone area, twice the power handling and
usually much better linearity that one woofer does. The box can be
half the volume too.

This idea is known as " push pull" or "Isobaric".


Thanks Phil, just looked it up. Seems like a good way to 'use up' my
two drivers.


** You should download WinISD and try it - it is just sooooooo
simple to use.

http://www.linearteam.dk/default.aspx?pageid=winisd

There is a large library of various brand woofers included so you can
immediately use the supplied TS data.

Also, and most importantly, you can vary box volumes and tuning
points at whim and instantly see what the effect is.

In most cases, major reductions in box volume are possible with only
small penalties in flatness and LF cut off points.



..... Phil

Just a heads up....I did find the TS data for my pair of Skytronic bass
drivers online and am looking at various WinISD box designs. According to
WinISD an 'optimum' isobaric+vented design is a mere 122.5 l which is
smaller than I expected.

Woof woof
 

On 28/04/2012 19:01, TonyL wrote:


Just a heads up....I did find the TS data for my pair of Skytronic bass
drivers online and am looking at various WinISD box designs. According to
WinISD an 'optimum' isobaric+vented design is a mere 122.5 l which is
smaller than I expected.

That seems quite high. Do you get 225l for a single driver?
An isobarik box should be half the volume of that for a single driver.

--
Eiron.

Woof woof
 

On 01/05/2012 11:23, Eiron wrote:
On 28/04/2012 19:01, TonyL wrote:


Just a heads up....I did find the TS data for my pair of Skytronic bass
drivers online and am looking at various WinISD box designs. According to
WinISD an 'optimum' isobaric+vented design is a mere 122.5 l which is
smaller than I expected.

That seems quite high. Do you get 225l for a single driver?
An isobarik box should be half the volume of that for a single driver.

Oops That should be 245l. Must be going senile!

--
Eiron.

Woof woof
 

Eiron wrote:
On 01/05/2012 11:23, Eiron wrote:
On 28/04/2012 19:01, TonyL wrote:


Just a heads up....I did find the TS data for my pair of Skytronic
bass drivers online and am looking at various WinISD box designs.
According to WinISD an 'optimum' isobaric+vented design is a mere
122.5 l which is smaller than I expected.

That seems quite high. Do you get 225l for a single driver?
An isobarik box should be half the volume of that for a single
driver.

Oops That should be 245l. Must be going senile!

Yes, 122.5 for isobaric and 245 for single driver.

Here's the TS data I used, culled from some online source:

Driver : Skytronic 902.222
Vas : 129.0
Qts : 0.49
Fs : 29.00
SPL : 88.50
-------------------------------------
Number of drivers : 2 (used as isobarik installation)
Box type : Vented
Box size : 122.5 l
Tuning frequency : 23.77 Hz
Vent : 1 vent(s)
0.273 m length for each
0.102 m round
---------------------------------------

This translates to a box of approximately W0.5m x H0.8m x D0.3m internal
dimensions. Seems not that huge to me considering the SPL -3dB level is at
about 22Hz. But my experience with enclosure design is zero (although I've
built a few from kits).

Woof woof
 

"TonyL"
Eiron wrote:

Just a heads up....I did find the TS data for my pair of Skytronic
bass drivers online and am looking at various WinISD box designs.
According to WinISD an 'optimum' isobaric+vented design is a mere
122.5 l which is smaller than I expected.

That seems quite high. Do you get 225l for a single driver?
An isobarik box should be half the volume of that for a single
driver.

Oops That should be 245l. Must be going senile!

Yes, 122.5 for isobaric and 245 for single driver.

Here's the TS data I used, culled from some online source:

Driver : Skytronic 902.222
Vas : 129.0
Qts : 0.49
Fs : 29.00
SPL : 88.50
-------------------------------------
Number of drivers : 2 (used as isobarik installation)
Box type : Vented
Box size : 122.5 l
Tuning frequency : 23.77 Hz
Vent : 1 vent(s)
0.273 m length for each
0.102 m round
---------------------------------------


** The TS numbers are typical for a budget woofer intended for
sealed box operation - so the " Isobarick" arrangement is ideal,
when you have two on hand.

Tuning a ported box to 24Hz is not advantageous as there is so
little program energy at that frequency. Box tuning corresponds
with a * large minimum* in the excursion of a cone for a given
SPL - so it is good to have that happen at a useful frequency.

Be better use a 75 litre box and tune it to 32 Hz - response is
then +/- 1dB from 105Hz to 32 Hz and down by 3dB at 28Hz.
A pair of 82mm x 345mm ports does the job.



.... Phil

Woof woof
 

Phil Allison wrote:

Be better use a 75 litre box and tune it to 32 Hz - response is
then +/- 1dB from 105Hz to 32 Hz and down by 3dB at 28Hz.
A pair of 82mm x 345mm ports does the job.


Thanks Phil...got the idea.

This is more of a fun/interest project for me. Not sure if I really need
more extended bass since my existing studio monitors came with individual
'calibration certificates' showing almost flat responses down to 50Hz. Might
be good for rattling the windows I guess.

Woof woof
 

"TonyL"
Phil Allison wrote:

Be better use a 75 litre box and tune it to 32 Hz - response is
then +/- 1dB from 105Hz to 32 Hz and down by 3dB at 28Hz.
A pair of 82mm x 345mm ports does the job.


Thanks Phil...got the idea.

This is more of a fun/interest project for me. Not sure if I really need
more extended bass since my existing studio monitors came with individual
'calibration certificates' showing almost flat responses down to 50Hz.
Might be good for rattling the windows I guess.


** Response curves alone prove nothing about a speaker's ability to produce
high SPLs at low frequencies. The latter game depends entirely on effective
moving cone areas and the use of tuned vents as sound producers.

Even in a typical domestic loungeroom, anything less than a 10 inch driver
in a ported box is a joke.

For use with action movies on DVDs, 15 inches is the minimum.

Size does matter.



..... Phil